In recent years, there have been much used in the broad field thermoplastic elastomers which are soft materials having a rubber elasticity, need no vulcanization process, and allow molding processing and recycling like thermoplastic resins.
For example, polymers of a conjugated diene monomer such as 1,3-butadiene and isoprene, or copolymers of a conjugated diene monomer with a vinyl aromatic monomer such as styrene copolymerizable with the conjugated diene monomer are very important as modifiers for impact-resistant transparent resins or polyolefins and polystyrene resins.
Hydrogenated polymers obtained by adding hydrogen to olefinic double bond moieties contained in the conjugated dienic polymers have a feature of being excellent in weather resistance. The hydrogenated polymers make the most of the feature and are used for automobile parts, household appliance parts, electric wire coverings, medical parts, sundries, footwear and the like.
The conjugated dienic polymer is generally produced by a living anionic polymerization using an alkyllithium or the like as an initiator. Further in the case of providing a hydrogenated polymer, after the polymerization, a hydrogenation reaction is carried out on olefinic double bond moieties by using a periodic table group VIII or IV metal as a catalyst.
Various methods for hydrogenating a polymer having olefinic double bonds are reported, and for example, a hydrogenation method is known which uses a catalyst of a combination of a compound of a periodic table group VIII metal, particularly nickel or cobalt, with a suitable reducing agent such as an alkylaluminum compound. Besides, a method for hydrogenating unsaturated double bonds of conjugated dienic polymers is known which uses a catalyst of a combination of a compound of titanium, which is a periodic table group IV metal, for example, a bis(cyclopentadienyl)titanium compound, with a suitable reducing agent such as an alkylaluminum compound.
As described above, thermoplastic elastomers, particularly conjugated dienic polymers and hydrogenated polymers thereof as described above, come to contain metal residues originated from a polymerization initiator and a hydrogenation catalyst. Since the metal residues in a polymer solution lead to decreases in various qualities including humps, rough surface, coloring and turbidity of products, the metal residues need to be removed efficiently in the production process.
Then, some methods of removing metal residues remaining in a polymer solution are proposed. For example, Patent Literature 1 discloses a method of removing a lithium residue in a polymer solution by vigorously mixing the polymer solution and water by using a rotary dispersing machine having a meshing structure.
Patent Literature 2 discloses a method of removing a residue of a periodic table group VIII metal including nickel by using an oxidizing agent and a dicarboxylic acid. Further Patent Literature 3 discloses a method of adsorbing in a silicate salt. Other than the above, the removal of lithium and periodic table group VIII metals has been disclosed hitherto in a large number of prior literatures.
On the other hand, almost no report has been made so far on a method of removing a titanium residue. For example, a technology of removing titanium and lithium residues by using an inorganic acid, an alcohol and water is disclosed in Patent Literature 4. Barely Patent Literature 5 discloses removing titanium and lithium residues by using an organic acid, an alcohol and water.